Alerts: trigger on approach — geofence radius = alert radius

- Region radius changed from fixed 300 m forecourt fence to the alert
  radius (the Alerts-tab slider), clamped to the device's
  maximumRegionMonitoringDistance.
- Notification now fires when you ENTER the winner's circle while
  approaching, not when you reach the forecourt.
- Cheapest-gate unchanged: still only fires when the entered station is
  the cheapest within the alert radius (fresh prices). Dedup still 1/
  station/hour. 35 tests pass.

FuelBoard/ProximityMonitor.swift

@@ -41,6 +41,9 @@ struct TieChoice {
 ///
 /// Region budget: iOS allows 20 monitored regions per app — favourites are
 /// always registered first, then the closest remaining stations fill the rest.
+/// Each region is a circle around a station with radius = the alert radius
+/// (clamped to the device ceiling), so the notification fires on APPROACH —
+/// entering the winner's circle — not at the forecourt.
 @MainActor
 final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLocationManagerDelegate, UNUserNotificationCenterDelegate {
     @Published var monitoredStationIDs: [String] = []
@@ -117,10 +120,17 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
         candidates.append(contentsOf: others)
 
         var registered: [String] = []
+        // Trigger radius = the ALERT radius (approach heads-up), not a tiny
+        // forecourt fence: entering the winner's region means you're within
+        // the alert radius of it, and the notification fires as you approach.
+        // Clamp to the device's region-monitoring ceiling — registering a
+        // larger region fails (kCLErrorRegionMonitoringFailure) or is reduced.
+        let maxRegion = manager.maximumRegionMonitoringDistance
+        let triggerRadius = maxRegion > 0 ? min(radiusKM * 1000, maxRegion) : radiusKM * 1000
         for station in candidates.prefix(18) where station.prices[fuel] != nil {
             let region = CLCircularRegion(
                 center: CLLocationCoordinate2D(latitude: station.lat, longitude: station.lng),
-                radius: 300,
+                radius: triggerRadius,
                 identifier: station.id
             )
             region.notifyOnEntry = true